1. Field of Invention
This invention relates to hydraulic valve lifters, and more specifically to variable duration hydraulic valve lifters, also know as fast bleed lifters.
2. Prior Art
Conventional hydraulic valve lifters have the benefit of adjusting the valve train, if necessary, every time the valve seats. They are designed with very little bleed down as compared with variable duration lifters. In fact, most hydraulic lifters are designed with as little bleed down as possible.
Variable duration hydraulic valve lifters, on the other hand, have a deliberately faster bleed down rate so as to improve the low end performance of the engine. They are an improvement upon the basic hydraulic lifter for increasing low end torque, engine vacuum and other low end performance characteristics of high performance engines. The performance cam of the high performance engine causes valve overlap, which is effective at improving performance at high speeds, but reduces low-end torque, engine vacuum, fuel economy, increases exhaust emissions and produces a rougher idle. The variable duration, or “fast bleed” lifter helps to improve this problem. The bleed down rate of a variable duration lifter is carefully set so that the plunger leaks down more at low rpm than it does at higher rpm. This begins to restore the lift and duration as the rpm increases; hence the term “variable duration”.
Jack L. Rhoads has three prior patents on bleed-down lifters, the first of which is U.S. Pat. No. 3,921,609, issued on Nov. 25, 1975. The second is a modification and improvement on the first and bears U.S. Pat. No. 4,524,731, issued Jun. 25, 1985. Thorough descriptions of the purpose and functioning of bleed-down lifters are found in those patents. Since variable duration lifters cause the valve train to tick at idle, a third U.S. Pat. No. 4,913,106, designed to quiet the valve train, was issued on Apr. 3, 1990. This patent requires the use of a specially modified camshaft. Our invention applies to these lifters, as well as any other fast bleed lifter such as the “sloppy fit” lifter in which the bleed bypass consists of a loosely fit plunger. It also applies to all the different designs of hydraulic lifters such as flat tappet, roller lifters, lash adjusters, hydraulic buckets, etc.
None of the above patents, however, produce the performance increases for conventional cams that this invention does. They either do not have the precise bleed down control necessary, or as in U.S. Pat. No. 4,913,106, the design is for a completely different purpose of quieting the valve train by utilizing a modified cam with special closing ramps. A conventional cam is a cam that is designed for use with conventional lifters, either hydraulic or solid lifters, and is not specifically designed to be used exclusively with fast bleed down lifters referred to in U.S. Pat. No. 4,913,106.
The former patents control the amount of bleed down by the size of groove machined into the side of the plunger, and other bleed down lifters such as the “sloppy fit” control bleed down by the dimensional clearances between plunger and the outer lifter body. They are adjusted from the top down, leaving ample travel between the plunger and lifter seat for excessive bleed down if the bleed rate is too high. This makes precise bleed down impossible as valve spring pressure, oil pressure, oil viscosity and engine temperature all work together to alter the total bleed down. A bleed rate that is ideal for one engine may be too much for another. As a result, the bleed rate must be made less than optimum because of the concern that too much bleed down will cause undue stress to the valve train which could cause valve train damage. The latter patent has provision for controlling the bleed down, but its purpose is to quiet the valve train by utilizing a modified cam with special closing ramps. It was not intended to be used with conventional cams at all, and virtually all cams currently available are of the conventional design.
This invention combines the precision bleed down control of the latter patent, with the conventional high performance cams used with the former two patents. As a result of this combination, performance gains have been significantly improved with camshafts that are readily available to the public, without excessive stress to the valve train. By limiting the bleed down of the variable duration lifters as described in this invention, maximum reductions in lift and duration can be realized without fear of bleeding too much and damaging the valve train. This has many advantages over the prior art.